The present application is related to a ceramic foam filter for filtering molten metal. More specifically, the present invention is related to a ceramic foam filter comprising a partial glass coating.
Ceramic foam filters are widely used in the filtration of metals. The filters are typically formed by a sponge replication technique wherein polyurethane foam is coated with ceramic slurry, then dried and fired. During firing, the polyurethane foam on the inside vaporizes and the ceramic particles in the slurry bond to form a contiguous network of ceramic struts resulting in an exoskeleton-like foam structure that positively replicates the original polyurethane foam.
The struts at the surface of ceramic foam filters produced using the sponge replication technique tend to be inherently weak. Small filter fragments easily release from the surface with little stress applied. These filter fragments, if dislodged, can eventually reach the casting, which is highly undesirable. To enhance the strength of the ceramic foam surface, manufacturers typically spray the filter surfaces with additional ceramic slurry prior to firing. This technique has been shown to reduce the amount of particles released from a filter prior to use, which may result from packaging, shipping, handling, impact, etc.
Spraying filters with ceramic slurry is a problematic process. One must use relatively fine spray nozzles to sufficiently atomize the slurry to a fine mist to produce a smooth, continuous coating. Because the shear rate through a fine spray nozzle is high, it requires slurry with low viscosity and shear thinning behavior. This in turn requires the use of relatively fine powder, which tends to be expensive and demands high water content. If the spray slurry does not have sufficiently low viscosity, or shear thinning characteristics, the nozzle will clog.
Filters are typically sprayed with slurry after the first drying step, which tends to soften the face of the filter. Therefore, it must be handled very carefully to avoid damage to the surface. An additional drying step may be required before inserting the filter into the kiln.
Spraying the filter also reduces the size of pore windows at the surface and may completely close at least some portion of the porosity. This can cause problems when molten aluminum is first being primed through the filter, and can inhibit flow of aluminum after it has primed the filter. If the average window size of the pores is too fine, or the volume fraction of porosity at the surface is excessively closed, the filter may not prime or may only partially prime, and may restrict flow to an unacceptable level. Because spraying alters the pore structure of the surface, non-uniform application of the coating can induce significant variation in the pressure required to prime a filter, the success rate of fully priming a filter versus partial priming, and the flow rate of metal from filter to filter.
There has been an ongoing desire in the art for an improved ceramic foam filter that avoids the deficiencies of the art.